A method of separating ions-implanted wafers after bonding to manufacture SOI wafers, i.e., an ion implantation separation method (a technique also referred to as the Smart Cut method (registered trademark)), has attracted attention as a method of manufacturing silicon on insulator (SOI) wafers, particularly a method of manufacturing thin-SOI wafers that enable improvement in performance of advanced integrated circuits.
This ion implantation separation method is a technique to form an SOI wafer (See Patent Literature 1) in the following manner: an oxide film (an insulator film) is formed on at least one of two silicon wafers; gas ions such as hydrogen ions or rare gas ions are implanted from a front surface of one of the silicon wafers (a bond wafer) to form a micro bubble layer (an enclosed layer) in the interior of the wafer; the surface from which the ions are implanted is then brought into close contact with the other silicon wafer (a base wafer) through the oxide film; a heat treatment (a delamination heat treatment) is then performed to cleave one of the wafers (the bond wafer) along the micro bubble layer so that the bond wafer is separated into a thin film; and another heat treatment (a bonding heat treatment) is then performed to strengthen a bond between the wafers. At this point, the cleavage plane (the separation surface) is a surface of an SOI layer and an SOI wafer having a thin SOI with high uniformity is relatively readily obtained. The ion implantation separation method can manufacture a bonded wafer by bonding the bond wafer and the base wafer directly without the insulator film.
The SOI wafer after the separation, however, has a damage layer on its surface due to the ion implantation. This surface is rougher than a surface of a normal mirror-polished silicon wafer. The ion implantation separation method accordingly needs to eliminate such a damage layer and surface roughness.
Conventionally, mirror polishing with extremely small polishing stock removal (a stock removal of about 100 nm), referred to as touch polishing, is performed to remove the damaged layer and so on in the final step after the bonding heat treatment. Even though the implantation of ions such as hydrogen ions and the separation can achieve the uniformity of the thickness of the SOI layer to a certain extent, a polishing process including such a mechanical factor performed on the SOI layer degrades this uniformity because of nonuniform polishing stock removal.
To solve this problem, a flattening process including a high-temperature heat treatment is recently performed to improve the surface roughness, instead of the touch polishing.
Patent Literature 2, for example, proposes performing rapid thermal annealing (RTA) of a heat treatment under a reducing atmosphere containing hydrogen without polishing a surface of an SOI layer after the delamination heat treatment or the bonding heat treatment. Patent Literature 3 in claim 2 etc., proposes forming an oxide film on an SOI layer by a heat treatment under an oxidizing atmosphere after the delamination heat treatment or the bonding heat treatment, then removing the oxide film (a sacrificial oxidation process), and then performing a heat treatment (the RTA process) under a reducing atmosphere.
In Patent Literature 4, oxidation induced stacking faults (OSFs), which are easy to occur when the separation surface is directly oxidized, are avoided by performing a sacrificial oxidation process after a flattening heat treatment is performed under an atmosphere of inert gas, hydrogen gas, or mixed gas thereof. In this way, the separation surface can be flattened and the OSFs can be avoided.
Patent Literature 5 discloses a bonding heat treatment performed by an oxidation heat treatment at a temperature of less than 950° C. and a subsequent heat treatment at a temperature of 1000° C. or more under an inert gas atmosphere containing oxygen of not more than 5%, in order to reliably prevent OSFs from occurring on the separation surface when the bonding heat treatment is performed under an oxidizing atmosphere to enhance the bonding strength of a bonded wafer after the separation.
Patent Literature 6 discloses a method of manufacturing a bonded wafer, including a first RTA process performed under an atmosphere containing hydrogen on a bonded wafer after the separation of a bond wafer, a sacrificial oxidation process performed to reduce the thickness of a thin film, and a subsequent second RTA process performed at a higher temperature than that in the first RTA process under an atmosphere containing hydrogen, for the purpose of providing a method of manufacturing a bonded wafer that can inhibit an increase in BMD density and flatten the surface of the thin film sufficiently when the thin film of the bonded wafer is flattened and thinned by the combination of the RTA and sacrificial oxidation processes.